This program of research is concerned with the ability of persons with Prader-Willi syndrome (PWS) to perceive correctly certain classes of stimuli that have been found to pose substantial perceptual problems for adults with mild mental retardation. They include global stereoscopic forms generated by random element stereograms, global motion of kinetic forms generated by random element kinematograms, and the integration of stimulus dimensions that define the metric of three-dimensional space. While these stimuli are simple physically, they induce complex percepts. Moreover, they are perceived quickly and automatically without engaging attention or other cognitive processes. These characteristics have inspired efforts to develop models and computer simulations of the underlying processes. That inquiry has revealed that these stimuli impose formidable information processing or computational demands on the perceptual system. The computational problems they pose, led my colleagues and me to the conjecture that mildly retarded persons may encounter in perceiving them correctly. Considerable evidence in support of that conjecture has been obtained from a series of studies devoted to elucidating the perceptual capacities of mildly retarded adults. The basic result is that the mildly retarded persons can process the stimuli well above chance when they are at maximum values with respect to energy and information. But modest reduction in those variables produces substantial impairments in performance. Under conditions where nonretarded subjects perform without error, retarded subjects perform at chance. The pattern of results from this line of inquiry supports strongly the hypothesis that the heretofore unsuspected perceptual deficits reflect fundamental impairment of visual cortical mechanisms. The present project assesses the perceptual capacity of persons with PWS using the same stimulus conditions as those employed in the investigation of the mildly retarded. These include discrimination of forms defined by motion, with variations in density and correlation, and sensitivity to environment cues to visual space. The same psychophysical methods will be employed using equipment already in place.